EE triple bonds (E = Group 13) promoted by charge transfer from alkali metals

We report herein evidence of EE triple bonds (E = Group 13) promoted by the perpendicularly surrounding M4 (M = Li and Na) motifs. The global minimum of E2M4 (E = B, Al, and Ga) is a D4h-symmetry structure where E-E is surrounded perpendicularly by square M4 motifs, whereas the Cs-symmetry structure formed by E-E-M surrounded partially by M3 motifs is the lowest-energy structure for E = In and Tl. Chemical bonding reveals that strong charge transfer from M4 to E2 dimer not only provides efficient electrons to form EE triple bonds, but also the resulting electrostatic interactions between M4 and E2 play a driving force to stabilize these intriguing triply bonded species.